The full stretch, Leonard; every other one gotcha! (6)
I believe the answer is:
length
'the full stretch' is the definition.
'length' can be an answer for 'stretch' (I've seen this before). I'm not sure about the 'the full' bit.
'leonard every other one gotcha' is the wordplay.
'leonard' becomes 'len' (I've seen this in other clues).
'every other' means one should take alternating letters.
The alternating letters of 'gotcha' are 'gth'.
'len'+'gth'='LENGTH'
Can you help me to learn more?
(Other definitions for length that I've seen before include "Something's largest dimension" , "Linear extent" , "A brief swim" , "End-to-end dimension" , "stretch" .)